Conventionally, there are widely used so-called compact disks (hereinafter called CD) wherein music information or other information is recorded as digital signals by the use of pits formed thereon. The reproduction from those CD's is performed by optical disk reproducing devices dedicated to reproduction.
Normally, there are pieces of information successively recorded on the CD's, and in reproduction, a reproducing operation such as successive reproduction or selective reproduction is performed with respect to a desired piece of information by collating absolute addresses predeterminately recorded on the disk with an absolute address indicating a recording start position of the piece of desired information, recorded in a TOC (Table Of Contents) area disposed at the innermost section on the disk.
Meanwhile, in recent years, magneto-optical disks as optical disks capable of recording, reproducing and erasing operations, which will take the place of optical disks such as CD's used only for reproduction, have been widely developed. In those magneto-optical disks, a thin magnetic film having vertical magnetic anisotoropy is usually used as a recording medium, and a laser beam which is focused to a spot of substantial 1 .mu.m is applied thereto in order to perform recording, reproducing and erasing operations. More specifically, the recording and erasing operations are achieved by effecting magnetization reversal by an external magnetic field by utilizing the fact that a coercive force is lowered because of a temperature rise in the spot where the laser beam is focused. On the other hand, the reproducing operation is achieved by utilizing the fact that a plane of polarization of the laser beam rotates by virtue of magneto-optical effect and by detecting the rotation of the polarizing plane by an analyzer.
For those disks of re-writable type such as magneto-optical disks or those of DRAW type recordable only once, it is desirable to provide an optical disk recording/reproducing device which has a standardized reproduction method with conventional CD's so as to be interchangeably applicable to any of those types in its recording or reproducing operation for music information or the like. In that case, rotation control by so-called constant linear velocity (hereinafter called CLV), which is performed in CD's and wherein relative velocity of an optical head along a track is kept constant, can be adopted for the rotation of optical disks in recording or reproducing operation. Further, conventionally so-called CD-ROM's are provided, wherein various data other than music information are recorded in CD's used only for reproduction, and the CLV control is also adopted for those CD-ROM's.
However, in the CLV control, it is necessary to vary the rotation velocity of a motor depending on positions on the disk in the radius direction, and therefore the rotation control of the motor becomes complicated. Moreover, since the rotation velocity of the motor must be changed when the optical head is shifted in a radius direction to access a desired track of the disk, required access time tends to become longer.
Moreover, in the CLV control, for example, in the case of reproducing information recorded in a CD-ROM and transferring it to an external device, the required reproducing time per one rotation of the disk increases as the relevant position gets closer to the circumferential section, and therefore a problem is presented in that the required reproducing time for the entire region of the disk tends to become longer.
In order to solve the above problems, it is proposed to adopt so-called constant angular velocity (hereinafter called CAV) control, whereby the rotation speed of the motor is kept constant regardless of positions on the disk when information such as data for computers wherein changes of time series are allowed is recorded or reproduced. However, in the CAV method, since a frequency of clock for recording or reproducing is kept constant regardless of positions on the disk, recording density lowers as the relevant position gets closer to the circumferential section, thereby arising the problem that enough recording capacity is not obtained.
For that reason, conventionally, it has been proposed to adopt MCAV (Modified CAV) method which has an increased recording capacity by increasing a recording frequency as the relevant position gets closer to the circumferential section in the CAV control method so as to make the recording density substantially constant regardless of inner sections or outer sections of the disk. When recordings are made by the MCAV method, however, it is necessary to gradually increase the frequency of reproducing clocks as the relevant position gets closer to the circumferential section also in reproducing operations.
In the conventional MCAV method, several kinds of frequency generators are provided, and in recording, a recording frequency generator for generating a suitable recording frequency for a disk area to be recorded therein, is selected among those frequency generators so as to perform the recording operation. In reproduction, one of those frequency generators suitable for a disk area to be reproduced therefrom, and a signal of the frequency generator is used as a reference signal in reproducing operation. In that case, there are a plurality of areas provided on the disk, each corresponding to one of those several kinds of frequencies, and a recording density of each area is kept substantially constant; however, compared with the recording by the CLV method, the equivalent recording density cannot be obtained.
Furthermore, in the case of recordable optical disks such as magneto-optical disks, pre-recorded information such as absolute addresses is predeterminately recorded thereon in an unerasable condition; however, a format whereby the pre-recorded information is recorded is normally different between those disks provided with CLV rotation control and those provided with CAV or MCAV control. Therefore, interchangeability between optical disks of CLV method and those of CAV (MCAV) method has not been achieved.